1. Field of the Invention
The present invention describes a method for functional testing of an ultrasonic sensor on a motor vehicle, a method for operating an ultrasonic sensor on a motor vehicle, and a distance measuring device having at least one ultrasonic sensor for use in a motor vehicle.
2. Description of Related Art
Distance measuring devices for motor vehicles which determine the distance from obstructions in the vehicle surroundings with the aid of ultrasonic signals have been known for quite some time. A sensor emits an ultrasonic signal which is reflected by an obstruction. The reflected signal is received by the sensor which has emitted the signal, or optionally by another sensor. The distance of the obstruction from the vehicle is determined based on the propagation time of the signal. Such ultrasonic distance measuring devices are used in particular to assist the driver in parking the vehicle. The uses of the driver assistance system range from outputting a simple warning message regarding a recognized obstruction to automatically controlling the speed as a function of the obstruction situation (park-stop function). Increasingly higher demands are being placed on the availability of the distance measuring device, and thus, of the ultrasonic sensors. It is therefore known to carry out functional testing, frequently also referred to as “blindness testing,” for the particular surroundings sensor system, in the form of self-tests. In particular for ultrasonic sensors, a gradual drop in performance may also result due to soiling.
A method is known from the published German patent application document DE 10 2005 057 973 A1 for functional testing of an ultrasonic sensor on a motor vehicle, in which the ultrasonic sensor emits an ultrasonic signal, and the ultrasonic signal is reflected by a ground surface in front of the vehicle and received by the ultrasonic sensor or another ultrasonic sensor. The duration and/or the amplitude of the emitted ultrasonic signal is/are selected to be large enough that reception of the signal that is reflected by the ground surface is made possible under customary conditions, and that functioning of the ultrasonic sensor is confirmed upon receipt of the reflected signal.
In comparison to the intended use of the ultrasonic sensor for distance measurement, also referred to below as “useful mode,” the duration and/or the amplitude of the emitted ultrasonic signal is/are accordingly explicitly altered for carrying out the functional testing. As a consequence, however, the functional testing must be carried out in a particular operating mode, referred to below as “test mode,” since otherwise, significant interferences and therefore errors would result during the distance measurement. As a result, however, all driver assistance functions which use the ultrasonic sensor are not available during the test mode. For functional testing which is meaningfully carried out before the start of the function, this results in a significant delay in the availability of the driver assistance functions. Since limited functionality or lack of functionality of the ultrasonic sensor may also occur during operation, for example as the result of icing or heavy soiling, functional testing during ongoing operation is also meaningful. However, in the known methods this is possible only during time periods in which the affected driver assistance function does not have to be available. For a classical parking function, this is the case, for example, for vehicle speeds above a speed threshold of 30 km/h. Due to the further functional development of ultrasonic sensors, for example the use for park-stop functions, it is increasingly difficult to find operating conditions, and therefore operating times, in which the driver assistance function, which uses the ultrasonic sensor to be tested, does not have to be available.